Gas filter devices protect their user from inhaling gases and vapors hazardous to health. This protection lasts until the adsorption capacity of the filter is exhausted. The time period from the start of the use of the filter to the point in time at which relevant concentrations of the component to be separated by the filter, i.e., concentrations that are harmful to the user of the filter, appear on the inhalation side, is called the retention time. The filter retention time depends on a number of parameters, e.g., the type and amount of the adsorbent contained in the filter, the design of the filter, the type and amount of the toxic substance, the environmental conditions, such as the temperature and the relative humidity, as well as the respiratory minute volume of the user. To make possible the effective protection of the user of the gas filter device from filter breakthrough, he must be warned well ahead of time to ensure that sufficient time is left for him to leave the hazardous area.
A gas filter, in which a toxic gas detector is arranged within the adsorbent bed to display the state of saturation of the bed, has been known from Utility Model No. DE 88 14 222 U1. The drawback of the prior-art arrangement is that only an absolute measurement can be performed with the prior-art toxic gas detector and a preliminary standardization of the toxic gas detector is therefore necessary to eliminate drift processes. However, such a standardization can be performed with difficulty only in the case of a toxic gas detector integrated within the adsorbent bed, because direct exposure of the detector to a test gas is not possible.